Aluminum and aluminum alloys are extremely valuable and have achieved wide spread use in all applications where light weight and high strength to weight ratio are important. The transportation industry including aircraft and other uses particularly find aluminum alloys highly useful. Many techniques of fabrication have been used for aluminum and aluminum alloys. One such technique is the superplastic forming technique. In order to fully take advantage of the desirable strength and weight properties of aluminum it is desirable in many instances to utilize superplastic forming techniques. Aluminum can be formed into complex parts by the additional technique of superplastic forming. By complex parts we mean parts having a shape of such complexity that they can not readily be formed by standard casting, molding, forging, machining and welding techniques.
Superplastic forming is typically used by forming sheet stock of between about 0.040 to 3/16 inches in thickness or more preferably between about 0.060 to 0.125 inches in thickness. In superplastic forming a die having a desired shape may be used. A piece of stock aluminum or aluminum alloy such as sheet stock is introduced into the die and pressure is exerted on one side of the stock and reduced on the other side to conform the shape of the sheet stock to the shape of the die. In another technique two or more sheets of aluminum may be bonded together and then inflated for example as held in a suitable die, as shown in FIG. 3. As the sheets are inflated the shape and thickness of the sheet material change and it becomes a more complex shape such as a honeycomb or other shape.
These superplastically formed shapes or parts can be used as structural elements or can be a final part in themselves. For example honeycomb sheeting may be used as structural elements and subject to additional fabrication when incorporated into the end products as a an aircraft frame or wing structure.
The superplastic method of forming allows these complex shape parts to be formed and manufactured in a way that can take advantage of the inherently high strength to weight ratios possessed by aluminum and aluminum alloys. However, there are limitations in this fabrication method. Particularly as concerns the use of aluminum and aluminum alloys, for example, it is difficult to produce laminated or honeycomb structures from aluminum sheets and plates since it is difficult to obtain suitable bonds between adjacent sheets by conventional techniques. The presence of the oxide on the surface of aluminum reduces the ability of aluminum parts to be bonded together to produce a strong bond. For example by the difusion bonding or rolling bonding method. Conventional bonding techniques interfere with the fine grain structure which is required to fabricate aluminum by the superplastic method. Welding or brazing techniques for example, due to the use of heat, disrupt the fine grain structure of the aluminum stock causing coarsening of the grain structure. The coarse grain structure destroys or severely restricts the ability of the parts to be formed by the superplastic method subsequent to such bonding techniques.
Other bonding techniques such as adhesive bonding also are restricted in their applicability to the superplastic forming method primarily due to weakness of the bond and difficulty of achieving the bond in the desired patterns in many cases. Where the bond is not sufficiently strong the peel stress on the bond area produced by superplastic forming may be sufficient to destroy the bond in the localized area and thus render the parts unsuitable for their intended use. While it is known to use clamping techniques at bond nodes to compensate for the inherent weakness of adhesive bonding, this technique is severely limited. It cannot be conveniently used for many large parts or structural elements and requires additional labor, set up time and capital requirements.
Removing oxide film on the bond area of aluminum sheeting prior to bonding by the diffusion method has also been tried but it also has not proved satisfactory particularly for production of larger parts and structural elements. The increased labor and capital cost required and the difficulty of achieving and maintaining clean, oxide free, surfaces throughout the bonding process has restricted the use of diffusion bonding.
Applicants have discovered an improved method of superplastically forming parts and structural elements from aluminum sheets and the like. Applicants are able to achieve a strong peel resistant bond in the joined aluminum elements by explosively bonding he elements together. Applicants are also able to overcome the grain disrupting effects of the explosive bonding technique and produce a joined structure having the fine microstructure necessary for superplastic forming for example by expansion into honeycomb structures. In particular, applicants have found that by subjecting aluminum stock to an annealing treatment followed by explosive bonding and then to a subsequent annealing treatment that they can produce superplastically formed aluminum parts which have superior bond strength, which do not require additional clamping operations to hold the bonded areas together during the expansion which occurs in superplastic forming and which do not require that the surfaces be free of the oxide film normally present thereon.
Applicants' are aware of the following U.S. patents the disclosures of which are incorporated by reference herein: U.S. Pat. Nos. 3,024,526; 3,066,390; 3,331,121; 3,344,510; 3,449,819; 3,543,382; 3,735,476; 3,927,817; 4,021,901; 4,264,029 and 4,415,375.
Applicants' method results in superplastically formed aluminum and aluminum alloy parts which are extremely light in weight. The parts or structural elements formed may be produced at higher production rates and at lower costs than would have been permitted by use of previous techniques. Moreover, the bonds between adjacent element in the super plastically formed parts or structures are stronger and more uniform.
The invention will be more apparent from the following detailed description of the preferred embodiments and the drawings.